In typical arc welding systems, for example for welding two sections of pipe together, a carriage is circumferentially mounted on one of the pipes, and a torch head moves along the carriage, circumferentially welding the pipes. A power supply generates a current which is conducted through a contact tip located within the torch head, to a feed wire electrode, across a gap, through the pipe and ultimately to ground.
As the current flows across the gap, an arc is created, establishing an arc voltage across the gap. The linear distance between the tip of the feed wire electrode and the work is generally referred to as "arc length". The feed wire electrode is continuously fed through the torch head towards the work, and the tip of the feed wire electrode continuously melts due to an intense heat generated by the arc. As the tip of the feed wire electrode melts, the molten metal is deposited on the work as filler metal.
A preferable mode of depositing the metal on the work is known as "spray transfer". The spray transfer mode of metal deposition results in molten metal particles being ejected from the melting tip of the feed wire electrode as the feed wire electrode is being fed towards the work. The ejected particles are transferred across the arc towards the work, where the particles are received by a molten puddle formed in the work.
The size of the molten puddle must be properly controlled or it can become too large to maintain control of the process, especially in out of position welding where gravity causes the puddle to flow. For a thorough discussion of spray transfer, see Kimbrough et al, U.S. Pat. No. 4,301,355 entitled "Gas Metal Arc Welding System", which has been incorporated by reference.
The size of the molten puddle has typically been controlled by pulsing the power alternately at a peak level and a base level. When the power is at the peak level, spray transfer occurs. When the power is at the base level, the arc is maintained, but the spray transfer is minimized, thereby allowing the molten puddle to chill and solidify.
Kimbrough et al '355 discloses a control system wherein the power is pulsed at a variable frequency and variable duty cycle. However, according to Kimbrough et al '355, any disturbances, such as a misaligned carriage, a jarring of the torch head relative to the pipe or a deformity on the pipe, will reduce the arc length and resultingly reduce the arc voltage. The rate at which the tip of the feed wire electrode melts, or wire burn-off rate, is proportional to the power delivered. Because the power delivered is directly proportional to the arc voltage, the reduction in the arc voltage reduces the wire burn-off rate. The reduction in the wire burn-off rate reduces the arc length, further reducing the arc voltage, and compounding the problem. Ultimately the tip of the feed wire electrode contacts the work, extinguishing the arc.